Ultra high molecular weight (UHMW) polyethylene is an ethylene polymer with an extremely high molecular weight of one million or greater characterized by high resistance to impact.
Ballistic resistant polymer monolayers, including inter alia polyethylene monolayers, are typically formed from fibers, a solution or a powder of the polymer. Polymer fibers are woven, knitted or not woven and monolayers composed from these fibers typically comprise an elastic resin or a polymeric matrix that encapsulate and holds the fibers together (see for example, U.S. Pat. Nos. 4,574,105; 4,820,568 and 4,944,974 among others).
The prior art teaches that the percentage of resin, bonding materials and the like must not exceed 20% of the total weight of a ballistic resistant material, otherwise the anti-ballistic qualities of the material begin to deteriorate. For example, European Patent No. 768,507 to van de Goot et al. discloses a ballistic-resistant article containing a compressed stack of monolayers containing unidirectionally oriented reinforcing aramid fibers and a matrix consisting of a polymer, the content of which is at most 25 weight percentage, the fiber direction in each monolayer being rotated with respect to the fiber direction in an adjacent monolayer. Ballistic resistant articles comprising successive layer of fibers in a matrix composition, the matrix composition is about 20 weight percentage of the total weight are disclosed in U.S. Pat. Nos. 5,552,208 and 5,587,230 among others.
U.S. Pat. No. 5,340,633 to van der Loo discloses a multilayered antiballistic structure comprising a first layer which comprises ceramic tiles, a second layer of composite material comprising polyalkene filaments, a matrix that surrounds the polyalkene filaments and an intermediate layer of a material between the first and the second layers, having a flexural modulus which is higher than that of the material of the second layer and lower than that of the ceramic material.
A ballistic resistant article constructed of high performance fibers and devoid of resins is disclosed in U.S. Pat. No. 5,935,678 to Park. The article includes two arrays of high performance, unidirectionally-oriented fiber bundles, cross-plied at an angle with respect to one another in the absence of adhesives or bonding agents. Thermoplastic films, including inter alia polyethylene films, are bonded to the outer surfaces of the arrays without penetration of the films into the fiber bundles. This arrangement substantially reduces the weight of the resulting article without compromising the anti-ballistic characteristics thereof.
Formation of UHMW polyethylene tapes and films from a powder of polyethylene rather than from polyethylene fibers is known in the art. U.S. Pat. No. 4,879,076 to Kobayashi et al. discloses a process for producing a polyethylene material of great mechanical strength and high elastic modulus from particulate UHMW polyethylene, wherein the particulate UHMW polyethylene is obtained from polymerization of ethylene at a temperature between 20° C. to 110° C. in the presence of a catalyst comprising magnesium, vanadium and an organometallic compound.
U.S. Pat. No. 5,091,133 to Kobayashi et al. discloses an improvement to the process disclosed in the U.S. Pat. No. 4,879,076 patent. The process is directed to continuous production of a high-strength and high modulus polyolefin material, comprising feeding a polyolefin powder between a combination of endless belts, compressing-molding the polyolefin powder in a temperature below the melting point thereof and rolling the resultant compression-molded polyolefin followed by stretching.
U.S. Pat. No. 5,106,555 to Kobayashi et al. discloses another improvement to the processes disclosed in the U.S. Pat. Nos. 4,879,076 and 5,091,133 patents, the improvement including: (i) using an UHMW polyethylene powder that has an intrinsic viscosity of 5-50 dl/g as measured at 135° C. in decalin as a principal component in the process and (ii) concurrently processing, in at least one of the compression-molding step and rolling step, an olefin polymer having an intrinsic viscosity from 0.5-3 dl/g.
Another improvement to the processes for continuous production of a high-strength and high modulus polyolefin material is disclosed in U.S. Pat. No. 5,106,558 to Kobayashi et al. The process comprises, prior to feeding and compressing-molding the polyethylene powder, the step of mixing an UHMW polyethylene powder with a liquid organic compound having a boiling point higher than the melting point of said polyethylene.
U.S. Pat. No. 5,200,129 to Kobayashi et al. discloses an alternative process for continuously producing a high-strength and high-modulus polyolefin material. The process comprises using, at the compression-molding step, particular pressing means comprising two opposing sets of a plurality of rollers, the rollers in each set are not connected together as disclosed in the U.S. Pat. No. 5,091,133 patent, rather each roller is rotatably supported at the shaft ends by a frame.
A split polyethylene stretched tape produced by subjecting UHMW polyethylene to compression-molding, rolling, stretching and then to splitting is disclosed in U.S. Pat. No. 5,578,373 to Kobayashi et al. The split polyethylene has a tensile strength of 0.7 to 5 GPa when twisted in the range of 50-500 times/m.
Despite the foregoing materials and processes, there still remains a need for polymeric materials having enhanced properties for use in antiballistic articles, and these are now provided by the present invention.